This invention is in the field of nuclear magnetic resonance (NMR) apparatus and relates to an adjustable plug-in wand to facilitate either single-frequency operation or multi-nuclei operation of the probe wherein the particular choice of nuclei is determined by the plug-in wand.
NMR spectrometers typically include a probe containing the sample to be analyzed, a superconducting magnet for generating a static magnetic field B0, and console unit containing the electronic equipment needed to operate the spectrometer system. The probe contains one or more radio-frequency (RF) coils surrounding the sample for generating time-varying magnetic fields B1 perpendicular to the static magnetic field B0. For multi-nuclear probes several different RF fields may be applied simultaneously or consecutively to stimulate the resonance of two or more nuclei which may be in the sample.
The multiple tuning of the probe is achieved with the aid of one or more additional RF coils and capacitors that are removed physically from the RF coils containing the NMR sample. Typically a spectrometer system is designed to detect protons and deuterium, which is used for set-up and spectrometer frequency control. These two frequencies may be provided by one RF coil by double tuning. Another coil disposed at right angles to this first coil may be double tuned to detect phosphorus-31 and carbon-13. It is often desirable to use the same probe to detect a different second set of nuclei, for example sodium-23 and chlorene-35. Existing probes are generally constructed to operate on a pre-selected one or two frequencies (in addition to protons and deuterium), therefore two or more probes are needed for applications requiring up to four additional frequencies.
U.S. Pat. No. 5,982,179 xe2x80x9cNMR Circuit Switchxe2x80x9d, assigned to the Assignee of the present invention, describes an NMR probe with a stepped cavity for locating switch components therein. For single frequency operation a capacitor switch is activated upon insertion of a capacitor stick. For double frequency operation a xc2xc wave stick incorporating a metallic threaded screw that closes a threaded connection switch thereby connecting capacitors internal to the probe and the xc2xc wave center connector of the switch to operate cooperatively to permit double frequency operation. To permit operation at different pairs of frequencies an extension stick is used in place of the xc2xc wave stick and an external section of wave tube is mounted on the extension stick wherein part of the wave tube is in the probe and part on the stick. For each pair of additional frequencies a different stick and extension tube is required.
There is therefore a need for a single probe design that facilitates single tune and double tune operation of any pre-selected set of nuclei, that is capable of operating at one or two pre-selected sets of frequencies.
It is a feature of the present invention to provide a probe with a set of wands, each of which provides the tuning for a single or for double frequency operation. The wands for double frequency operation have one or more capacitors and a central conducting rod with an adjustable conductive collar having spring finger contacts that in cooperation with a conducting tube form an adjustable xc2xc wave stub and produce double tune operation. The tube may either be fixed to the wand or to the probe. Wands for different set of double frequencies contain capacitors with different values and with the conductive collar set to different positions.
In one embodiment of this invention, the wands for different single frequencies or different sets of double frequencies may be identical in their construction making them easier to construct. The values selected for capacitors within the wand, the way of their connection, and the position of the adjustable collar determine the different frequencies. Single frequency operation is obtained by different electrical connections within the wand. In a preferred embodiment clips hold the capacitors, permitting them to be changed. An electrical jumper may be used in place of a capacitor, or the clip may be left vacant thereby, changing the circuit configuration.
The insertion of the wand into the probe requires no turning or rotating. The wand is inserted directly in the probe: the end of the wand plugs into a keyed electrical socket within the probe. The electrical plug and socket permits changing the probe configuration and operating frequencies, without requiring the wand to be rotated. All the wands can be of the same size and length and still have means for providing for different resonant frequencies of the xc2xc wave shorted stub.
An additional feature of the present invention is the low manufacturing cost of the probe and wand, as the mechanical parts of all probes and wands are identical. The electrical socket within the probe and mating electrical plug on the wand provides a low cost method of electrically coupling the wand to the probe.